1. Field of the Invention
The present invention relates in general to digital noise filters and in particular to a digital noise filtration system and method for reducing noise in digital data, such as compressed data.
2. Related Art
Data can be encoded and represented as compressed data so that less information (fewer bits) is used to store, transmit, or process the data more efficiently. After the compressed data is stored, transmitted, or is processed, it is decoded to obtain the original data. Since fewer bits represent the compressed data, less storage space is required to store the data and less time is required to transmit the data from one location to another. Thus, current data compression systems and methods are very important, useful and popular.
One popular type of data encoding system is JPEG compression (Joint Photographic Experts Group). JPEG encoding is usually employed for compressing digital image files. JPEG compression involves removing the least visible information in the image and can reduce the size of an image file by up to 100 times (100:1 compression ratio). The compression ratio of JPEG encoders is user selectable. The compression ratio is directly related to the quality of the image.
Typical JPEG encoders have four main steps. First a matrix of pixels, such as an 8.times.8 pixel block, is extracted from the image. Second, a discrete cosine transform (DCT) is calculated for the block. Third, coefficients of the DCT are rounded off by quantizers according to the desired compression ratio and the specified image quality. Fourth, the quantized coefficients are compressed using an encoding scheme such as Huffman coding or arithmetic coding. The final compressed code is then written to an output file or transmitted.
Although JPEG compression is an acceptable compression scheme for most compression ratios, problems arise with high compression ratios and highly compressed data. Namely, "halos" or "darkening effects" occur after high compression ratios are used for JPEG encoding. "Halos" or "darkening effects" are light gray patches surrounding text and other high-contrast areas in highly compressed JPEG data. In black text on white backgrounds, the light gray patches should be white areas. When printed, these gray patches result in the appearance of plural dots around the text, making it look fuzzy.
Since the color facsimile standard requires that each page be sent as JPEG data, documents with halos are prominent in color facsimile transmissions. For example, when image data is compressed enough so that a 4.times.6 inch photo can be sent in one minute, text looks exceptionally poor, primarily because of the surrounding halos. Therefore what is needed is a system and method that reduces halos and reduces stray dots around text in highly compressed data. Therefore, what is needed is a system and method for reducing unwanted noise in digital data. What is also needed is a system and method for reducing halos in encoded JPEG data.
Whatever the merits of the above mentioned systems and methods, they do not achieve the benefits of the present invention.